Known manufacturing methods for a stretched thermoplastic resin film include the sequential biaxial stretching method, in which an unstretched thermoplastic resin film is first stretched in the length direction to obtain a uniaxial stretched film and then the obtained uniaxial stretched film is stretched in the width direction by introducing it into a tenter oven and the simultaneous biaxial stretching method, in which an unstretched thermoplastic resin film is introduced into a tenter oven and stretched in both the length and width directions simultaneously.
Stretched thermoplastic resin films are used in a wide range of industrial material applications, including packaging. Of all those films, sequential biaxially stretched films of polyester, polyolefin and polyamide resins are, because of their excellent mechanical characteristics, thermal characteristics, electrical characteristics, and so on, particularly widely used and enjoying growing demand in areas where unstretched films are of little use.
However, a tenter oven for manufacturing a stretched thermoplastic resin film has the problem of accompanying air flow induced by running of the film and the problem of a flow of air having a different setting temperature into an adjacent chamber or flow of outside air of the tenter oven into the inside thereof, due to, among other things, an imbalance between the flow rates of heated air blown into the tenter oven and the exhaust air ejected from it which causes incomplete air circulation in each of the chambers in the tenter oven. Each problem is a phenomenon of air flow in the direction of running of the film across the boundaries of the chambers. Such air flow is called “MD” (abbreviation of Machine Direction) flow.
When an MD flow occurs, air having different temperature enters from the outside of a chamber flows in the vicinity of the film and is mixed with heated air blown out of a air blowing nozzle provided inside the chamber. As a result, the film is exposed to a significant temperature unevenness. The temperature unevenness in the width direction of the film is a potential cause of unenvenness in film thickness and characteristics, and can therefore not only decrease product quality, but also reduce productivity by causing film tearing inside the tenter oven.
If flows of air at different temperatures mix into a chamber due to an MD flow, namely, if, for instance, air that is cooler than the temperature setting of the circulating air of the chamber mixes into the circulating air, the amount of steam consumed by the heat exchanger to reheat the circulating air to the temperature setting of the chamber increases, thus reducing energy efficiently. Triggered by the accompanying air flow and an imbalance between the supply and exhaust air flows, the blown air loses the ability to flow in a straight-line path towards the film surface and becomes prone to be dragged over in the running direction of the film. Once this situation develops, the MD flow increases, and the blowing nozzles fail to fulfill their heating performance potential. To maintain the heating performance under this condition, it becomes necessary to increase the flow rate of blown air so as to heat the film until the temperature needed for stretching, etc. is reached, leading to an increase in electricity consumption by the heat exchanger.
Known methods aimed at solving the above problem include one that increases the flow rate of blown air along the edges of the film compared to the middle to reduce the temperature unevenness in the width direction of the film (JP 05-096619 A) and one that controls the heat exchanger according to temperatures detected by temperature sensors to evenly heat the film in the width direction and thus reduce temperature unevenness in the width direction (JP 10-249933 A or JP 2002-018970 A).
Known air blowing nozzles that are relatively immune to the influence of the MD flow include a perforated panel-type air blowing nozzle that comprises a number of circular holes provided over the air blowing surface in an array pattern to circumvent the influence of the MD flow by dispersing the air leaving the air blowing surface (JP 2009-255511 A).
There is also a method to suppress the fluttering of a sheet by employing a nozzle featuring a flat section and adjoining sloped sections that is designed to blow parallel flows of air onto the surface of the sheet. That method is known to have an effect of suppressing the heat inflow and outflow at the entrance and exit of the heat treatment chamber by allowing the gap between the sheet and the nozzle to be narrowed (JP 2005-008407 A).
A method to minimize the unevenness of the width direction distribution of heat transfer efficiency by employing an air blowing nozzle comprising multiple circular holes is also known. The air blowing holes are arranged in two rows, at an identical interval Py in each row but in a staggered fashion between the rows. The first and second rows are spaced at interval Px. The air blowing surface and the sheet running plane face each other across distance L, and the air blowing holes on the air blowing surface have diameter D. Intervals Px and Py, distance L and diameter D satisfy the following formula (1): 5≦(L/D)/(Px/Py)≦9 and formula (2): 4≦L/D≦8 (WO 2008/114586 A1).
However, although the methods described in JP 05-096619 A, JP 10-249933 A and JP 2002-018970 A are effective in cases where air circulation is self-contained inside each individual chamber, has no temperature equalizing effect against the temperature unevenness caused to a certain chamber by an inflow of air from the adjoining chamber with a different temperature setting from the temperature setting of the certain chamber.
The method described in JP 2009-255511 A, which adopts a perforated panel-type air blowing nozzle, is prone to develop an temperature unevenness attributable to the distribution of blowing holes, making it necessary to optimize the blowing hole arrangement pattern, namely the hole diameter, hole interval in the running direction of the film, hole interval in the width direction of the film, number of rows, and the like. For this reason, it incurs large monetary and time costs from the designing of the arrangement pattern of the blowing holes of the air nozzle to its application to production.
The method described in JP 2005-008407 A aims to primarily ensure a stable running of the film and is not specifically targeted at the heating, cooling or drying function of a tenter oven. Namely, it is only useful as a supplementary measure to secure the full capability of the film heating (or cooling or drying) nozzles in a tenter oven, so that the MD flow reduction effect, as such, of the nozzle described in JP 2005-008407 A is small.
When a nozzle of the design described in JP 2005-008407 A is installed to face the upper face of the film passing plane, along with another facing the lower face of the film passing plane, the effect of the method described in JP 2005-008407 A to suppress film fluttering cannot be obtained due to an interference between the Coanda effect brought about by the nozzles, namely the film run stabilization effect afforded by the air blowing pressure and suction force, that is present above the film passing plane and the same effect that is present below the film passing plane. For this reason, JP 2005-008407 A specifies that the nozzle described should be installed only on one side of the film passing plane.
Just as the method described in JP 2009-255511 A, the method described in WO 2008/114586 A1 has an effect of making the blown air relatively immune to the influence of the MD flow that flows across the tenter oven, but it does not have any effect of completely shutting down the MD flow. For this reason, concern remains as to the occurrence of unevenness in physical properties in the width direction of the film or an increase in tenter oven energy consumption as a result of the MD flow.
It could therefore be helpful to reduce the temperature unevenness of the film and make it possible to manufacture a stretched thermoplastic resin film with uniform characteristics and thickness in the width direction by suppressing the MD flow generated inside the tenter oven, as well as providing a tenter oven capable of reducing the energy consumption needed to heat the film to a predetermined temperature and maintain it at that temperature.